Initial acceleration of the beams in the LANSCE linear accelerator at Los Alamos National Laboratory is still presently accomplished through the use of two 750-keV Cockcroft-Walton (CW) based injectors. To reduce longterm operational risks and to realize future beam performance goals, plans are underway to replace the existing H CW injector with a modern replacement, 4rod Radio-Frequency Quadrupole (RFQ) based front end. Significant technical progress has been made since we last reported on this project. Status and progress of the design and fabrication of the RFQ, the RF system, beam transports, and integrated accelerator test stand will be discussed.

Status of the lansce front end upgrade

A Full-scale prototype of a new 201.25 MHz RF Final Power Amplifier (FPA) for Los Alamos Neutron Science Center (LANSCE) has been designed, fabricated, assembled and installed in the test facility. This prototype was successfully tested and met the physics and electronics design criteria. The team faced design and manufacturing challenges, having a goal to produce 2 MW peak power at 13% duty factor, at the elevation of over 2 km in Los Alamos. The mechanical design of the final power amplifier was built around a Thales TH628 Diacrode{sup R}, a state-of-art tetrode power tube. The main structure includes Input circuit, Output circuit, Grid decoupling circuit, Output coupler, Tuning pistons, and a cooling system. Many types of material were utilized to make this new RF amplifier. The fabrication processes of the key components were completed in the Prototype Fabrication Division shop at Los Alamos National Laboratory. The critical plating procedures were achieved by private industry. The FPA mass is nearly 600 kg and installed in a beam structural support stand. In this paper, we summarize the FPA design basis and fabrication, plating, and assembly process steps with necessary lifting and handling fixtures. In addition, to ensure the quality of themore » FPA support structure a finite element analysis with seismic design forces has also been carried out.« less

Mechanical design and fabrication of a new rf power amplifier for lansce

As part of the modernization of the Los Alamos Neutron Science Center (LANSCE), a digital low level RF (LLRF) system was designed. The LLRF control system was implemented in a Field Programmable Gate Array (FPGA) using embedded Experimental Physics and Industrial Control System (EPICS) Input Output Controller (IOC) under the Real-Time Executive for Multiprocessor Systems (RTEMS). Proportional-Integral (PI) feedback controller, static beam feedforward controller, and iterative learning controller are implemented on the FPGA. The closed loop system performance was tested with a 10mA peak current proton beam. INTRODUCTION The modernization of the LANSCE 201 MHZ RF systems including the LLRF control systems, RF amplifier systems, water-cooling systems and networking is under way[1,2]. Analog low level RF control and electronics have been replaced with FPGA based control systems. The legacy LANSCE LLRF system was an analog PI Feedback control system which provided amplitude and phase error of < ±0.12% and ±0.1°, respectively. However, it does not support network control or data acquisition of LLRF parameters. The new design of the LLRF system using an FPGA with an embedded softcore processor and network support. Also, the FPGA-based design gives the ability for algorithm and processor modification and upgrade. In this note, an overview of the new LLRF system and the control system performance with a baseline proton beam of LANSCE are addressed. LLRF SYSTEM OVERVIEW The LLRF control system is illustrated in Figure 1. It consists of the clock system, analog front end, down converting mixers, FPGA (Stratix III) based control system, the RF power amplifier chain, and a cavity. The clock system generates the FPGA clock and LO signal from the 10MHz reference signal. The mixers are located in the LINAC tunnel, which down-converts the 201.25 MHz cavity and reference RF signal to the 25.15625 MHz intermediate-frequency (IF) signals. These IF signals are filtered by diplexer filters to eliminate the higher order harmonics and fed to 16-bit ADCs (LTC2208) where four times oversampling scheme is used. The ADC output of the reference IF signal is conditioned and is used for IF modulation of the base band I/Q control signals. The I and Q stream outputs of the ADC of the cavity IF signal are processed to remove the DC offsets and then they are filtered by 150-tap linear-phase low pass FIR filter, yielding baseband cavity field I and Q signals. Meanwhile, the relative phase of the cavity field with respect to the reference is calculated using CORDICs (Coordinate Rotation Digital Computers) from I and Q data of the reference IF signal and I and Q data of the cavity IF signal. The PI control algorithm is applied to the cavity field I and Q signals to obtain the control actions that will be digitally modulated onto control IF signal. A 16-bit DAC (AD9726) transforms the digital control IF signal back to analog IF signal and the analog IF signal is up-converted to the 201.25 MHz control RF. The analog control RF signal is amplified through the RF amplifier chain to the cavity. The LLRF control system parameters are loaded to memory mapped-registers located in the FPGA through the EPICS IOC over the network. For the register reads and writes, and waveform uploads, a CPU softcore, NIOS II processor, is embedded in FPGA and the EPICS IOC on the operating system Real-Time Executive for Multiprocessor Systems (RTEMS). Figure 1: Schematic block diagram of the digital field control module (FCM). ___________________________________________ # skwon@lanl.gov Proceedings of IPAC2016, Busan, Korea WEPOR044 06 Beam Instrumentation, Controls, Feedback and Operational Aspects T27 Low Level RF ISBN 978-3-95450-147-2 2771 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s FEEDBACK CONTROL PERFORMANCE Because of the high power requirement of the LANSCE accelerator, two diacrodes are installed and their outputs are combined through a hybrid to drive a DTL tank[2]. The performance of the feedback control system is shown figure 2. The PI feedback is applied. The amplitude and phase stabilities are ±0.05% and ±0.05°, respectively. Note that with higher PI gains, better performance is obtained. However, because there is a loop delay of a few μsec, in order to choose the gains it is necessary to consider the output performance and robustness, input usage and noise sensitivity. In order to confirm the performance of the feedback control, the amplitude and phase are measured with external monitors based on ADL5511 RF amplitude detector and AD8302 Phase Detector. The obtained signals are shown in figure 2 and the amplitude and phase stabilities on the flattop are ±0.04% and ±0.02°, respectively. Figure 2: Amplitude and Phase Stabilities of the Feedback System. (a) FCM measured amplitude, (b) FCM measured phase, (c) ADL5511 measured amplitude, (d) AD8302 phase detector measured phase. BEAM LOADING COMPENSATION BY FEEDFORWARD CONTROLS Two types of beam loading compensation feedforward controllers are implemented. The first beam feedforward controller is the static beam feedforward controller (SBFFC), where the detected beam current is read-back to the LLRF system and a proper amplification and rotation of the detected beam current generates the feedforward I and Q control signals. Figure 3 shows SBFFC performance when 10mA production beam having 20usec ramp is operating at 1 Hz rate on LANSCE DTL Tank 2. It is observed that the stabilities on the flattop are ±0.20% and ±0.07°, respectively. It is noted that around the middle of the beam ramp (10usec), the beam loading effect on the amplitude is not suppressed well (0.58%) though it is improved a lot compared with the case when only feedback controller is applied (Figure 4). This may be improved by a new scheme where the SBFFC enable time is adjusted optimally. Figure 3: Amplitude and Phase Stabilities of the Static Beam Feedforward Control. (a) ADL5511 RF amplitude detector, and (b) AD8302 Phase Detector. (c) Schematic of SBFFC. Green: Beam Current, 10mA. Figure 4: Amplitude stability Comparison. Feedback Only System has 2.3% transient stability. Green: Beam Current, 10mA. 0 100 200 300 400 500 600 700 800 90

Fpga Implementation of a Control System for the LANSCE Accelerator

The LANSCE RF system consists of four RF stations at 201 MHz and forty-four klystrons at 805 MHz. In the LANSCE accelerator, the beam source is injected into the RF system at 0.75 MeV. The beam is then accelerated to 100 MeV in four drift tube linac (DTL) tanks, driven at 201.25 MHz. Each 201 MHz RF system consists of a train of amplifiers, including a solid state amplifier, a tetrode, and a triode. After the DTL, the beam is accelerated from 100 MeV to 800 MeV in the forty-four coupled cavity linac (CCL) tanks at 805 MHz. The machine operates with a normal RF pulse width of 835 microseconds at a repetition rate up to 120 Hz, and sometimes operates with a pulse width up to 1.2 milliseconds at 1 Hz. This RF system has been operating for about 37 years. This paper summarizes the recent operational experience. The reliability of the 805 MHz and 201 MHz RF systems is discussed, and a summary the lifetime data of the 805 MHz klystrons and 201 MHz triodes is presented.

LANSCE 201 MHZ and 805 MHZ RF System experience

This paper gives an overview of recent results and future prospects on RF sources for linac applications, including klystrons, magnetrons and modulators.

Development and Future Prospects of Rf Sources for Linac Applications

Long pulses of high proton current (greater than one millisecond at 17 mA) at LANSCE are limited by the duty factor (DF) of the 201.25 MHz RF system powering the 100 MeV DTL. The operating point of the existing triode final power amplifiers (FPA) is approximately 75% of the published maximum anode dissipation rating, but is at 90% of the allowable limit with a margin of safety for tuning excursions, voltage standing wave effects and electron tube manufacturing variations. A new RF power system is under development for increasing both peak power and DF. The FPA uses a Thales TH628 Diacrode  with pyrolytic-graphite grids. It doesn't require the large anode modulator of the present triode system. Lower mains power consumption and reduced water-cooling requirements are some advantages of this scheme. A TH781 tetrode is being tested as an intermediate stage, using a Thales cavity amplifier. When installed, the new system will reduce the total number of electron power tubes from twenty-four to seven for the LANSCE DTL. Aspects of the RF modeling, mechanical design, RF testing, and other system considerations will be presented.

/pdf/a-new-201-25-mhz-high-power-rf-system-for-the-lansce-dtl-2wf3etl2q7.pdf

A new 201.25 mhz high power rf system for the lansce dtl

Acceleration of long pulses of proton current at high repetition rates (1 mS, 120 pps) at LANSCE have been impacted by reduced capability of the original 201.25 MHz RF system driving the 100 MeV DTL. The present operating point of the existing triode power amplifiers doesn’t allow margin for tuning excursions with voltage standing wave effects or for tube manufacturing variations. This has led to the undesirable consequence of operating LANSCE at half of its original duty factor for the past seven years to maintain acceptable beam availability. A recent program has been initiated to restore the accelerator to its original high current operation. Investigated alternatives for high power RF included the development of physically large klystrons, solid-state and new power grid tube (tetrode) amplifiers. The power grid tube was favored as it reused much of the existing infrastructure including water-cooling systems, coaxial transmission lines, high voltage power supplies and capacitor banks. High duty factor tetrodes called Diacrodes ® , are capable of supplying the DTL requirements with ample headroom. A LANL-designed power amplifier using the TH628L Diacrode® has produced 3 MW peak and 300 kW of mean power with > 14 dB of power gain and > 60% efficiency. It has been operating for over seven thousand hours generating RF power with a test load. Installation of the first new system begins in January of 2014.

SYSTEM CONSIDERATIONS FOR 201.25 MHz RF SYSTEM FOR LANSCE

The current LANSCE LLRF system is an analog PI Feedback control system which achieves the amplitude and phase error within 1% and 1 degree. The feedback system receives the cavity amplitude and phase and the crosstalk between the amplitude and the phase paths is significant. We propose an In-phase (I) and Quadrature (Q) based feedback control system which easily decouples the crosstalk of I and Q channels. A gain scheduled PI feedback controller with optimally generated set point trajectory reduces the transient peak of the feedback controller and hence reduce the fatigue of the RF amplifier chain. An additional feature of the controller is the Neural network based self-tuning PI feedback, where the neural network tunes the feedback gains to minimize the errors in the least squares sense. The proposed control system is implemented with Altera Stratix II FPGA. The control system is modeled with DSP Builder and automatically generates HDL. Altera SOPC Builder is used for the hardware integration of the DSP Builder model, memories, peripherals, and 32 bit NIOS II embedded processor. NIOS II processor equipped with real time operating system communicates with the host computer via Ethernet, uploads data, computes parameters, and downloads parameters. The proposed control system is tested with the low power test-stand for the robustness of the algorithm.

Gain scheduled neural network tuned pi feedback control system for the lansce accelerator

The LANSCE RM project is restoring the linac to it’s original high power capability after the power grid tube manufacturer could no longer provide triodes that could consistently meet our power requirements. High duty factor Diacrodes® now supply RF power to the largest DTL tank. These tetrodes reuse the existing infrastructure including water-cooling systems, coaxial transmission lines, high voltage power supplies and capacitor banks. The power amplifier system uses a combined pair of LANL-designed cavity amplifiers using the TH628L Diacrode® to produce as much as 3.5 MW peak and 420 kW of mean power. A digital low-level RF control system was developed to complement these new linear amplifiers. Design and testing was completed in 2012, with commercialization following in 2013. The first installation is commissioned. The two remaining high power RF systems for tanks 3 and 4 will be replaced in subsequent years using a hybrid old/new RF system until the changeover is complete. Features and operating results of the replacement system are summarized, along with observations from the rapid-paced installation project.

RESULTS FROM THE INSTALLATION OF A NEW 201 MHz RF SYSTEM AT LANSCE

A new 201.25 MHz RF system is being developed for the LANSCE proton drift tube linac (DTL). A planned upgrade will replace parts of the DTL RF system with new generation components. When installed for the LANSCE-R project, the new system will reduce the total number of electron power tubes from twenty-four to seven in the RF power plant. The 3.4 MW final power amplifier will use a Thales TH628 Diacrodereg. This state- of-the-art device eliminates the large anode modulator of the present triode system, and will be driven by a new tetrode intermediate power amplifier. In this mode of operation, this intermediate stage will provide 150 kW of peak power. The first DTL tank requires up to 400 kW of RF power, which will be provided by the same tetrode driver amplifier. A prototype system is being constructed to test components, using some of the infrastructure from previous RF projects. High voltage DC power became available through innovative re-engineering of an installed system. A summary of the design and construction of the intermediate power amplifier will be presented and test results will be summarized.

Progress on new high power RF system for lansce DTL

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